This invention relates to a laser scanner flaw detection systems, and more particularly to a method and apparatus for the inspection of wide web materials by inspecting such materials using a plurality of laser beams which are sequentially scanned over the material using a plurality of aligned synchronized rotary scanners.
In U.S. Pat. No. 3,980,891 entitled "Method and Apparatus for a Rotary Scanner Flaw Detection System", which is assigned to the assignee of the present invention, flaws are detected in the material being examined by repetitively scanning a suitable source of radiation, such as a laser beam, across the surface of the material. The laser beam is directed at a multifaceted rotating mirrored reflector drum which scans the material with a highly collimated scanning radiation beam. The laser light is reflected, transmitted or scattered from the material depending upon the characteristics of the material, which light is collected by a receiver having a suitable detector such as a photomultiplier tube. At any given instant of time during the scan, the photomultiplier output varies with the reflectivity, transmissivity or scattering properties of the spot of radiation on the material upon which the laser beam is impinging, and accordingly deviations from characteristic variations provide a means for indicating material flaws. One form of receiver which may be utilized in this system is shown and described in U.S. Pat. No. 3,900,265 entitled "Laser Scanner Flaw Detection System" which is assigned to the assignee of the present invention.
Another rotary scanner flaw detection system is shown and described in U.S. Pat. No. 3,866,054 entitled "Defect Size Discriminator Circuit for Web Inspection System" in which the receiver comprises a radiation conducting rod which conveys transmitted or reflected radiation from the beam to a photomultiplier tube positioned on the end of the rod. A diffusing strip is positioned in the rod so that when radiation is applied thereto from the material being inspected the radiation is dispersed within the rod causing internal reflection therein thereby transmitting the radiation through the rod to the photomultiplier tube positioned on the end thereof. After signals have been generated in accordance with the intensity of the radiation applied thereto from the material being examined, the signals are processed in an electronic processing circuitry to identify flaws in the material.
The electronic processing circuitry for laser inspection systems have become very sophisticated providing a variety of information about the nature and locations of detected flaws. Accordingly, the electronic processing circuitry will normally be the most expensive part of the system.
In order for the systems to function properly the laser beam must be scanned across the full width of the web of the material being inspected. Accordingly, as the web width increases the height of the scanner above the web increases in order to provide a complete scan across the web. Also as the distance between the web and the scanner increases, the spot size of the laser beam increases thereby reducing system resolution. For example, the normal distance between the scanner and the material being examined for webs up to 120" would be approximately 10'. In order to cover a web of up to 240" with the same scanner, it would have to be elevated in an additional 10' above the web which would double the spot size. Elevating the scanner to such a height may also not be feasible because the facility where the web is produced may not accommodate such a separation simply because enough space is not available or machinery or other super structure near the ceiling would interfere with the positioning or the scanning operation. Furthermore, the heat which is generated in the processing plant, for example, a paper mill would be intense near the ceiling and interfere with the proper functioning of the laser scanner. Alignment problems between the scanner and the receiver also become a problem at such distances.
Furthermore, if reduced spot size and better resolution are required even for narrower webs, such spot size and resolution would be limited by the distance which the scanner must be placed from the web in order to completely cover it with a scan.